The New ACS Calibration Pipeline: Putting the Electrons Back Where They Belong

Linda Smith, lsmith@stsci.edu, and the ACS Team

The Advanced Camera for Surveys (ACS) team has recently released a new version of the data calibration pipeline, calacs. The new version includes corrections for charge-transfer inefficiency (CTI) and the electronic artifacts introduced by the repair of the Wide Field Channel (WFC) during Servicing Mission 4 (SM4). Users will now see new data products when they retrieve ACS data from the Barbara A. Mikulski Archive for Space Telescopes.

 

Pixel-based CTI correction

Since the ACS was installed on the Hubble Space Telescope ten years ago, the continuous exposure to the harsh radiation environment of space has caused CTI in the charge-transfer device (CCD) detectors. CTI is due to damage in the silicon lattice, which traps electrons as the charge is being read out. This trapping produces images with trails of deferred charge extending away from bright objects. The left side of Figure 1 shows an example.

CTI compromises science with ACS because it alters the photometric, astrometric, and morphological characteristics of both faint and bright sources, particularly those located farthest from the readout amplifiers of the CCD, as well as sources with low background illumination.

Recently, significant advances have been made in correcting ACS/WFC images for CTI at the pixel level. Massey et al. (2010) showed that CTI trails could be successfully removed by fitting exponential decay parameters to hot pixel trails in science images. Subsequently, Anderson & Bedin (2010; AB10) extended this work to lower background levels using an empirical model to describe the trails of hot pixels in stacked dark exposures. Jay Anderson describes the details of this pixel-based correction in a recent Newsletter article (Anderson 2011).